Barbieri Thesis - BioMedical Materials program (BMM)

Chapter 7 – Polymer molecular weight and instructive composites
In calcium phosphate containing composites, the polyester molecular weight would
determine the fluid uptake and indirectly guide the overall degradation. We hypothesize
that composites containing amorphous polyesters with low molecular weight will
absorb more fluids than those with higher molecular weight. A more rapid polyester
hydrolysis would then generate rougher surface topographies with larger exposure of
apatite particles, which in turn would enhance the release of calcium and phosphate
ions. In addition, higher body fluid absorption will enhance biomolecular transport onto
and into the biomaterial resulting in higher chances of protein adsorption and surface
mineralisation, which play roles in recruiting and inducing (stem) cells to differentiate
into osteogenic phenotypes. It is therefore expected that the polyester molecular weight
in composites with calcium phosphate influences their bone forming ability.
To test this hypothesis, two polymers having same monomer chemistry but different
molecular weights were used to prepare composites with the same calcium phosphate
apatite content. As seen in Chapter 6, composites having (excessively) high L–lactide
monomer content degraded slowly and were not osteoinductive, whereas those
containing less L–lactide monomer, degraded more and triggered heterotopic bone
formation. Consequently, in this study we choose poly(D,L–lactide) as polymer phase of
composites to study the effect of molecular weight on their osteoinduction. Once the
composites were prepared with extrusion, we characterized their physicochemistry and
in vitro performance in terms of degradation, surface mineralisation and serum protein
adsorption. Thereafter we evaluated in vivo mineralisation, degradation and
osteoinductive potential after intramuscular implantation of the composites in dogs for
three months.
Besides this, the polymer molecular weight also influences on the viscoelastic properties
of the resulting composites. Polymers with low molecular weight are composed by
shorter chains that have more freedom to slide past each other when mechanically
stressed. Under the same filler content and same monomer chemistry, this fact would
lead to composite materials with lower stiffness and higher damping capacities as
compared to those containing higher molecular weight polymers. Therefore, we
evaluated also the effect of molecular weight on the dynamic mechanical properties of
the two materials over a physiological frequency sweep to simulate the natural cyclic
stresses occurring in bone.
7.2. Materials and methods
7.2.1. Apatite preparation and characterization
Nano–apatite powder was synthesized by adding (NH4)2HPO4 (Fluka, Steinheim,
Germany) aqueous solution (c=63.12 g L –1 ) to Ca(NO3)2·4H2O (Fluka) aqueous solution
(c=117.5 g L –1 ) at the controlled speed of 12.5 mL min –1 and 80±5ºC, with the reaction
pH kept above 10 by using ammonia (Fluka). After precipitation, the resulting powder
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